87 TBI to Carb conversion
 

Anybody take this on? Good Idea? recommended parts? What can I expect to pay in parts and labor?

I have an 87 with a 305. It currently has a Edelbrock intake and Holley TBI. It once ran strong, but we can't figure out what's going with it.. it dies when hit the gas. It's now stumped two mechanics.

http://lh3.ggpht.com/_Lf7i4POM-J4/S2...rockIntake.jpg

http://lh6.ggpht.com/_Lf7i4POM-J4/S2...olley502-5.jpg

Can I stay with my Throttle Body Intake? I suspect not. Should I go 4 barell?

More pics in my project link below.

Thanks!

Re: 87 TBI to Carb conversion
 

I'm in the mix of this swap in my crew cab but with a TBI'd 454 to carb. I've picked up a carb, intake, MSD distributor, some twin flo-masters & I'm hoping to get a single feed gas tank that I'll be using a electric holley blue fuel pump with.

I think the 305 will respond the very same way. Don't forget to remove the computer from under the dash .... that controls the fuel, distrib., all those xtra wires after you remove those parts.

You'll end up removing some extra vacum stuff too.

Ratty 46

Re: 87 TBI to Carb conversion
 

Did any one bother to perform a simple fuel pressure check. 9/10 TBI 'stump on the gas' problems can be traced to a low fuel pressure. Low fuel pressure can be due to tired pump, clogged fuel filter or bursted in tank fuel line - between FP and outlet port. These one are tough to spot sometimes as fuel gets warmer leak increases starving fuel injectors from fuel.

If your engine fires up and idles ok, but begins to stumble of idle then there is no good reason to tare off top half of it and replace it with venturi mixer. Today's techs are spoiled by advance diagnostic aids, but they can not think through a problem most of the time.

Going to a carb is bad, bad idea :(

//RF

Re: 87 TBI to Carb conversion
 

Don't give up on the TBI yet. You need to data log the engine while it is running and see what is going on. Without that info any mechanic is just guessing at what is wrong.

All you need is a data cable, a laptop and a piece of free software to read the data with like TunerPro RT.

http://www.tunerpro.net/

The guys at thirdgen.org have a bunch of great info on DIY PROM tuning and data logging is part of it.

Re: 87 TBI to Carb conversion
 

The fuel pump, filter, tank and gas were all replaced. Fuel pressure is fine, but we did have rust in tank at one point. Mechanic pulled up the TBI and says injectors are shooting smooth and consistent.

This last mechanic checked that the IAC (Idle Air Control) sensor was clean and will check the gas filter again.

My check engine code has alwas read error 34. Map Sensor voltage low.

We are thinking of changing out the Throtlle body Position Sensor (TPS).

By the way, :) can somebody confirm exactly where the TPS and IAC value and MAP sensors are exactly on my truck on this grid?

Many Thanks!

http://lh4.ggpht.com/_Lf7i4POM-J4/St...neGrid.gif.jpg

http://lh6.ggpht.com/_Lf7i4POM-J4/Sm...0/CIMG1776.JPG

Re: 87 TBI to Carb conversion
 

IAC
http://cds.activant.com/C2C/C01/10/454/6704992.jpg
http://cds.activant.com/C2C/C01/49/831/625575308.jpg

TPS
http://cds.activant.com/C2C/C01/32/143/1039856095.jpg
http://cds.activant.com/C2C/C01/37/934/591314349.jpg

MAP
http://cds.activant.com/C2C/C01/13/793/733469576.jpg
http://cds.activant.com/C2C/C01/73/732/929258905.jpg

Snuck another question in on me. MAP sensor is on a bracket right above the passenger side valve cover.

I know the TPS is on the passenger side of the TBI unit itself, 3 wires, I can see it on your bottom photo. The wires go up towards the breather assembly. I have the Holley TBI as well, and the TPS was bad on the brand new unit. Now I think I'm fighting dirty gas tanks since the fuel filter looked so nasty.

First I gotta fix a valve cover though before I can keep trying on my TBI. I'm very close though to yanking off the new Holley and putting the stock TBI back on. I got the same intake, but a 350 with more ponies, and my mechanics are saying it is throwing way too much fuel compared to the stock TBI, running very rich all the time.

Re: 87 TBI to Carb conversion
 

hmmm, it seems you and Randy are both having map sensor issues and you both have the Holly TBI.....coincidence??

In my experience with this era of TBI, nothing works better than stock parts.

Re: 87 TBI to Carb conversion
 

$400+ mistake on my part. All I could see was the BIGGER BETTER FASTER MORE HP! part. After the install all I see is JUNK DON'T USE STAY AWAY WASTE OF MONEY! part.

Re: 87 TBI to Carb conversion
 

How did they check the fuel pressure? You do not have a test port. You need to add a gauge fitting to the supply line next to the TBI unit and connect a test gauge to it. Put the test gauge on the cowl panel where you can see it while driving. I taped mine to the windshield. Do not put it inside the truck whatever you do.

As RF stated the pump may be fine when cold and first started but may lose pressure when it warms up. I have had two do this to me. I was able to see the pressure drop once the pump warmed up.

Re: 87 TBI to Carb conversion
 

:lol:
It happens to us all, AFAIK anyway :lol:

Re: 87 TBI to Carb conversion
 

Locate and a take a close look at your MAP sensor. It should have three wire connector (usually with a green tab) with the following three wires:
PIN C Gray +5Volt supply
PIN B Light Green MAP signal (to ECM)
PIN A Purple (or brown or sometimes tan) Sensor Ground

Disconnect connector, with ign on, engine off, measure voltage at Pin C. It should read 5 volts. If there is no +5Volts at PIN C look for harness breaks or bad connection between wire and pin inside connector.

The other common fault is open circuit at PIN B - sensor output never reaches ECM.

Changing TPS at this time is a waist of money. You can verify if TPS is functional without removing it from TB. Just disconnect harness connector and measure resistance between C & B. At idle throttle position you should see about 4.5 kOhm and at WOT should be about 0.9 kOhm (or 900 OHms) resistance should change smoothly as you move throttle shaft between the two extremes, Final check is measure between PIN A & C - you should measure about 4 kOhm.

To do these simple checks requires DVM - any parts store should have one for less than $20. Learn how to troubleshoot and post your findings.

//RF

Re: 87 TBI to Carb conversion
 

I bought the truck with all this after market stuff. Truthfully It was running great when I bought it, though I know the previous owner had just replaced the fuel filter.

To me, my problems started after I did an engine wash. I likely f***ed up the map sensor or something else.

By the way we already replaced the EGR valve as it had some kind of vacuum leak.


@RF. Thank You! That image is going in my project site right now!. I do need to learn how to troubleshoot the truck if I'm going to keep it.. though everybody I've talked to locally thinks TBI is a nightmare to figure out.. even if you are a so called mechanic.

Thanks guys.

Re: 87 TBI to Carb conversion
 

JC - you welcome. A side note - MAP sensors are easily damaged by moisture. It is conceivable that when you washed engine bay some moisture got into MAP sensor. Unfortunately I am some 2600 miles to the west and I have a bunch of MAP sensors that I pulled from JY expeditions (as well as other stuff). Functionality of MAP sensor can verified with a hand-held vacuum pump and DVM. This test is a bit involved, perhaps I should put together a DIY test on the TBI swap thread. No fancy test equipment is requried. That is the beauty of TBI - it is a very simple EFI system.

The PO installed the best upgrade possible - I would keep it.

//RF

Re: 87 TBI to Carb conversion
 

Update: My mechanic found an old Camaro from around the same year in the junk yard and swapped out the Throttle body keeping the injectors. Though I'm yet to see it he's says it's running strong and no longer dies when you gas it. Before doing that, he tried swapping all the compentents discussed - no changes resulted from changing the MAP Sensor, TPS or IAC.
He says the computer on the truck is not original.

I'm not clear yet on exactly why, but he had to disconnect some temperture control sensor (?) to keep it from from idleing way too high. However, when he disconnects it the engine light comes on .. no other apparent consiquence yet. Would you guys know what part he might be talking about? Even with it disconnected, he's finding it to idle kind of high.

He's across town so I can't even see it until Wednesday.


Many Thanks!

http://lh4.ggpht.com/_Lf7i4POM-J4/St...neGrid.gif.jpg

Re: 87 TBI to Carb conversion
 

In your last photo, to the left of the waterneck (towards the passenger side), that's a temp sensor. Goes in the front of the intake right by the water neck.

I got a new fuel tank, new fuel pump, new pickup filter and tossed the old stock TBI on mine. Runs like a champ now. Can't wait to get the other tank done now.

Re: 87 TBI to Carb conversion
 

22-V on the grid? I don't think my mechanic is able to tell if it's sending or not.

Re: 87 TBI to Carb conversion
 

Correct on the 22-v grid. Is a cheap sensor, we replaced mine when we dropped the new MAP sensor and such in it while fighting the rough idle and other issues.

Re: 87 TBI to Carb conversion
 

These are great threads. I'm still sneaking up on my TBI on my '87 454. RF has been a HUGE help and the truck seems to run better everytime I make a change (still haven't replaced the connectors/pig tails, but hoping that will go within the next couple of weeks). Thanks guys and keep up the great informational posts.

I'm not going to let TBI beat me!

Re: 87 TBI to Carb conversion
 

Camaro used a slightly different ECM (1228746) than a truck (1227747). If you look at ECM module there is a service number printed on a label along with broadcast code. Broadcast code (BCC) is used to identify emission - drivetrain calibration information stored in ECM's EPROM. For sake of discussion early TBI Camaros and truck C/K ECM's share same circuit design except that Camaro also incorporates IAT (Inlet Air Temperature sensor) on ECM pin C12 and there are two additional control lines: a) Charcoal Canister Purge (CCP) on pin A3 and b) AIR diverter control valve on ECM pin C1. You can swap 305 TBI truck into 305 F-body and it should run without any issues (provided that the two use the same trany). Swapping Camaro ECM into truck will result in DTC 23 being set (with open circuit ECM input line is pulled high which corresponds to very cold temperature -40C! ECM will command higher idle if it sees very cold air). All of this can be fixed with a custom EPROM - but that's another story all together.

Yep - it works, just have to follow step by step approach in troubleshooting. And it's simple enough to work on.

//RF

Re: 87 TBI to Carb conversion
 

Guys - I do really appreciate all your valuable feedback.

I'll preface this by saying I'm clearly no mechanic. So it's going to seem like I'm wasting my time, money and space a bit.

Update: I got the truck back. The guy that helped me out with it is a good guy, but I'm not sure he had everything he needed to really solve the problem. Honestly, not sure he did much but to take wild stabs in the dark frankly.

He replaced a Holley Throttle Body with some factory TBI he found. He says the model number matches the OEM for my truck. He also told me the temp sensor (which is brand new and you guys pointed for me) is disconnected now and I learned so is the Map Sensor (which is a swap). He said and (I confirmed) , the truck will not run with either connected at this point. :( I have a bag with all my old parts.

The truck is running, but clearly things are not entirely right. That said, the truck has never run l or idled as well since I've had it this past year.


When I picked up the truck last night it was out of gas and it was dark, so when I would hit the gas it looks like the pump would suck air and choke. If I kept one foot on the accelerator and hit the gas it would feel pretty strong.So I had it towed home (AAA - It has no tag) and added fuel on the trip home. This morning, It started right up and drove it. It has not driven that well in months.

I uploaded a video of this test drive. I'm sorry, it's annoyingly bad with the camera focusing on the steering wheel and gauges for too long. But will give you a picture of how it's running.


I'd like to also go back and append to my original question, Would it be a terrible idea to mount a Carb on top of existing intake? I have a 3704 EdelBrock TBI intake. Even possible? Is there an adapter? too restrictive? What carb? This guy agreed to do the install cheap. :) I understand I can get rid of all that electrical/sensory and mysterious crap, but I will need to add an external fuel pump.

Many Thanks again for any ideas or suggestions!

Re: 87 TBI to Carb conversion
 

The Edelbrock TBI intake sucks and adding a carb adapter to it will only make it worse if you could even find an adapter to put a carb onto it.

If you are dead set on going to a carburetor I would get a new carb intake for the center bolt heads. You may be able to find a used one but I don't believe Chevy made very many of them. The only one I can think of is the 87 & 88 Monte Carlo & El Camino that were still running Q-Jet carburetors when everybody else went to TBI. There are aftermarket ones out there but aren't very common as a used item.

Re: 87 TBI to Carb conversion
 

Subscribed! I have a 454 TBI I will have to figure out the rough idle on soon.... Good info here! :metal:

Re: 87 TBI to Carb conversion
 

JC

1) That 'mechanic' is a hack at best, but least he installed some different parts to see what works, what does not. Stay away from him - he has no clue how TBI operates.

2) By disconnecting coolant temperature sensor (CTS) ECM sees high CTS voltage which is equivalent to very cold temperature -40C. At -40C ECM will call for very rich mixture - this why you feel the power from engine.

3) By disconnecting MAP sensor ECM is forced into a limp mode (SES light is on continuously) Without MAP sensor input ECM uses TPS as its only source of engine load information available. It's ability to control fuel delivery is very limited.

4) TBI camaros were equipped with similar LO3 engines. So, if he really pooled TB unit from camaro it should have correct size injectors -2x 50lb-Hr which is adequate for a stock 305. (I would verify this soon, but its ok for now)

5) The hissing noise you are hearing is probably (It tough to be certain without being there) from a fully open, stuck IAC. This would explain very high idle 1100 to 1200 RPM.

6) I am also guessing that your 'mechanic' also screwed around with base timing by moving from 0 deg to 20. In limp mode ECM fixes timing to a constant value. He probably over advanced timing to get it to run without MAP and the moment MAP is reconnected ECM tries to control timing by advancing as it normally would which way too much - ECM has no idea that base timing is way off - killing engine in the process.

Analysis

a) Fuel system appears to deliver enough fuel.
b) Sensors MAP, TPS, CTS - remain questionable, but appear functional
c) IAC - operational - should be verified.

---------------------------------
Things to do

1) Reconnect all sensors plug CTS back in, reconnect MAP, make sure all other hoses, plugs are fully plugged in.

2) Perform POST = Power on self test. Move ignition key from Off to Acc to Run position - SES light should go from Solid ON - blink off - solid ON (do not start engine) Fuel pump should turn on for about 2-3 seconds and shut off. This would indicate that ECM is operating normally by being able to read EPROM contents. - Turn off ignition

3) Locate ALDL connector - it is located just under steering column. It looks like this:
http://www.aldlcable.com/uploads/carconnector.jpg

4) Pulling DTC codes with a simple paper clip. Insert paper clip into socket pins A & B (usually upper right corner) shorting the two togther. Turn ignition into run position, but do not start engine.
http://www.techedge.com.au/vehicle/im/con12pin.gif

Observe SES light. If there are no DTC codes set SES will blink code 12 three times. If there are DTC codes set each will be flashed 3 times in ascending order, pause followed by the same sequence again. So, for example of CTS and MAP are disconnected DTC 15 and 33 will be set:

12...12...12....15...15...15....33...33...33 12 12 12 15 15 15 33 33 33 and so on.

At the same time ECM will command IAC to fully closed position, which you can verify by observing IAC pentacle closing idle air passage - from the top looking down (sorry, I do not have photo handy right now - will post tomorrow)

Perform this simple test and post what you see (and please stay away from that 'mechanic') .

//RF

Re: 87 TBI to Carb conversion
 

Thanks RF,

Sensors back on. I do hear the gas pump or something turning on.

I think I'm reading 15 and 34? I might need to do it again as there might be more codes. All of that and see how it runs with sensors on.

Re: 87 TBI to Carb conversion
 

JC

You are in relatively good shape. You have codes 15 and 34 (unless you have additional codes). In your last video you've started your engine with A+B shorted (note rapidly flashing SES light). This is a limp mode - ECM ignores sensor inputs and works from a fixed set of values. It is not advisable to drive a truck - it is design for emergency operation only. As for the codes we know what they are:

15- coolant sensor or circuit/ low temp indicated
34- manifold absolute pressure (map) sensor signal voltage low

1) Lets clear all those DTC codes as they were artificially induced. This is easily done (engine off) by disconnecting negative Battery terminal for about 30 seconds. ECM will clear its fault memory from these codes until it sees subsequent fault conditions.

2) After clearing ECM fault memory we need to make sure that IAC (Idle air) is correctly reset and knows where it is. The procedure is very simple:

a) Place ECM into diagnostic mode by shorting pins A&B - this will force ECM to command IAC to continuously move into fully close position.
b) Without starting engine, ignition in run position you should be able to hear clicking noise from IAC being jammed into full close position. You can see IAC pentacle fully closing idle air passage in the TB (10 o'clock).
c) Remove the connector (It is 4 wire arranged in box) from the IAC and start engine while monitoring idle speed. Minimum idle speed for a stock engine should be around 450 rpm (can be adjusted).
d) Shut of engine, remove A+B jumper, re-connect IAC connector. Restart engine - idle should be around 600-650 RPM. Provided that base timing is correctly set and there are no vacuum leaks.

3) Take a look at the base of TB - there should be a (black) gasket between it and intake. I have bad feeling that it is a used one and it's leaking as I doubt that 'mechanic' installed a new one when he installed F-body TB on. It is worth replacing just to make sure. While looking at TB write down its OE part number - it is stamped on passenger side vertical ledge. You can see it in this photo (17093030):
http://i226.photobucket.com/albums/d...Picture007.jpg
from this part number we'll be able to determine which base gasket we need to get.

3) You'll need a timing light and distributor wrench to adjust base timing.

http://www.northerntool.com/images/p...9094234_lg.jpg

Without this wrench getting to distributor clamping bolt in the back of the engine is real PITA. It is also possible that your 'mechanic' did not tightened and distributor can be easily turned. Check it out.
I'll have to get a timing light - you must have friends that are gear heads?? Borrow, beg or rent one for now. I'll make no further comments about having no tools - everyone's situation is different.
If you are able to secure timing light and twist distributor (aka dizzy) setting timing is easy. You'll have to locate EST connector - it is a single wire connector (usually tan/black or brown/white wire) in the harness near Power brake booster.

This connector by-passes the electronic spark timing controlled by ECM and allows the initial timing to be adjusted by rotating the distributor. Use the following procedure (become familiar with entire procedure before beginning):

The ignition timing marks are located on the engine's front cover. A big notch slot on the balancer indicates engine is at (TDC) Top Dead Center. Adjust ignition timing as follows: You truck is specified at 0 TDC base timing

<> Connect a timing light to the number one (1) spark plug wire - it is closest to the front on drivers side. Connect the timing light power lead to a 12 volt power source - Battery terminal.

<> Start engine and allow to warm to operating temperature. Bring engine to idle (650-800 Rpm).

<> Disconnect EST connector - idle speed will drop down (it is normal)

<> Point timing light at the balancer while rotating (slowly) distributor to set mechanical timing. Look for slot on balancer to line-up against biggest notch on the timing tab. It is 0 degrees

<> Tighten distributor clamp to hold in position when initial timing is obtained.

<> Recheck timing.

<> shut off engine and reconnect EST connector. You'll have to disconnect battery for about 30 seconds again as DTC code 42 will be set by ECM - this is normal. After clearing ECM re-start engine. It should idle at about 650 RPM and if you check timing it will be bouncing between 18 and 22 degrees as ECM will be providing continuous timing control.

Alright - can handle this???? Questions??? No need to be afraid. BTW your videos are superb.

//RF