Help my 1987!!

[Gregs71]

I did a lot of searches but didn't come up with anything real solid so I thought I'd ask. The truck is a 1987 3/4 ton 4x4 with TBI.

What I am describing only happens after the truck is warmed up. When I start to step on the accelerator the motor starts to sputter. When it sputters I get a loud popping / backfire noise coming from the engine compartment (I might say from the dr side, but the sound could be traveling from somewhere else). There is also a slight backfire from the tailpipe. If I let it idle or coast it seems to be ok. If I put it in neutral and rev it up, it seems to be ok, too.

Does that mean its lean? I am leaning towards the 'coolant temp sensor'?

I bought a conde reader, but I only got Code 12 which means everything passed?

Help me out guys, this engine popping scares me.

Thanks, Greg

[f.monroe]

Plug wires breaking down once heated up ? Sounds like it is crossfiring or something . Frank

[Gregs71]

why would they break down after getting warm? Why / what would cause the popping from the motor??

Greg

[manimal]

Could be a burnt valve. Do you have or HAD any vacuum leaks? OR have you checked you EGR valve? They arent suppose to open when its cold(if I am thinking right) but when it warms up maybe it is allowing too much exhaust in.????? Good Luck.
-Lance

[Gregs71]

Would the EGR valve show up on the code reader??

Keep the ideas coming.

Greg

[bikerbomber]

I dono about 87 electrical systems, but if your truck is wired for a TBI I imagine its similar to my 88 TBI....Going on this assumption.......

Check your timing and your wires. Backfiring is detonation(somewhere along the way out). So check your ignition system.

Once the truck has reached operating temp, the engine coolant sender and the O2 sensor change it to a "closed-loop" system (at least it does on my 88 TBI) Adjusting fuel/air ratio according to the O2 and the engine temp.... I would check your O2 sensor and the engine temp sensor......

If that isn't it, run with what you know....it only happens after warm up....stay on that and it will narrow things down.

Oh and it "might" show on a code reader....that and the backfire wouldn't be caused by the egr
Good luck.

[James McClure]

Gregs: This could take some time to explain. First, on yours EGR won't set a code directly, BUT, it will set a lean code if it's stuck open (code 44) IF, the only code your getting is a code 12, EVERYTHING in the computer system is OK, or at least within parameters. I want you to check the obvious first, easy, quick tests. How is the idle speed when warmed up? If it's high, carefully spray a little carb cleaner around the base of the TBI unit. TBI base gaskets go all the time. Next get a bottle of windex and with the eng at idle in gear (parking brake set right?) spray each ign wire then the cap. If it starts skipping, replace the cap, rotor & wires. Use GM cap and rotor and make sure you transfer the ground tab to the new cap. Last test. GM had a problem with soft cams. This test will check for that. Warm eng, put it in gear, mash the brake, then throttle up hard. If the cam is bad, you'll get a constant pop, pop, pop up into the intake. If you find this to be true, make sure you do a compression test, wet & dry. DO NOT COMPROMISE, do the compression test!! If you want to talk about it I'll send you my phone number by e-mail. Address is jmcclur2@nycap.rr.com Let me know how you did. There is NO SUCH THING as a dumb question ok?

[jdurant]

I agree, check for an intake/vacuum leak. And anything that can do with air/fuel. What should the o2 counts be guys? I have seen them go bad on other cars and be just bad enough to make it run like crap, but not bad enough to throw a code. But yeah, it could be the cam. I just couldn't imagine wiping a lobe in my stock truck just out of the blue though.

[ChevyTech]

Sounds like heat could be causing the distributor module to break down. Many stores like NAPA in my area will test the module for free if you bring it in.

Make sure when you install or reinstall the distributor module that you use heat sink compound/grease under it. The grease helps dissipate heat from the module.

When you are in the distributor, see if the shaft will move sideways. On TBI distributors the housing/upper bushing area gets so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

Would the EGR valve show up on the code reader??

The EGR valve can cause problems without setting a trouble code. Generally your truck would only set an EGR code if the EGR valve would not open or the passages to the EGR were plugged.

[Gregs71]

Alright guys, because its cheap I am going to replace the o2, coolant temp sensor and the plugs/wires (if they need it) and go from there.

CHEVYTECH-- Do I have to remove the dist to see if the bearings are shot, or....?????

I am concerned about possibly doing some damage to the motor during this "trial and error" period. What do you think, do I risk blowing up my motor while 'trying to fix it'??

Thanks guys...

Greg

[Gregs71]

Something else I wanted to let you guys know. It was smogged in the middle of December. The first line is at 15 mph, the second line is at 25 mph. Here are the results:

HC
Max ave meas
173 54 115
108 41 108

CO
max ave meas
1.95 .25 .48
2.25 .20 .67

NO(ppm)
max ave meas
2268 549 0191
2185 475 0174


I hope this helps you guys, help me.
Thanks again,
Greg

[Kixwy2]

Greg, definitely listen to James. He gave me some excellent advice in troubleshooting my own problem, which sounds similar to yours. I do have some bad cams and am ultimately replacing the motor...More so because I don't know the history on the truck and can't take the PO's word...

[71/454]

My first thought was the EGR or the EGR solenoid. Just try unplugging the vacum line to the EGR, you'll get a ses light but if it stops acting up you have found it.I would do this first because it costs nothing to test it. I hate throwing parts at a truck, just look at my thread.

[James McClure]

Hay Chevy tech, I forgot about that. Now you mention it, I replaced tons of those distributors for just that problem. Gregs, when you have the cap off the dist just grab the rotor and try moving it side to side. If it moves enough for you to percieve it, get a new one. Distance between reluctor and pole piece is very small and you won't need much for it to fail. Be careful when you reinstall the cap. It MUST fit into the slots on the ign module. Hope this helps, jim

[ChevyTech]

CHEVYTECH-- Do I have to remove the dist to see if the bearings are shot, or....?????

You don’t need to pull the distributor. Just remove the cap and grab the rotor and see if the shaft is loose. Like James said.

I am concerned about possibly doing some damage to the motor during this "trial and error" period. What do you think, do I risk blowing up my motor while 'trying to fix it'??

When an engine is backfiring and popping in the exhaust it can do damage. I have seen mufflers blown open, carburetors scorched and melted, and if a back fire makes it backwards through the PCV valve it can ignite crankcase fumes that can blow the rocker covers and oil pan.

It depends how bad it is backfiring and popping in the exhaust as to how much it would concern me. If it is bad it should be easier to find the problem.

Originally Posted by James McClure

Hay Chevy tech, I forgot about that. Now you mention it, I replaced tons of those distributors for just that problem…

Yes it’s a common failure, and not only on Chevys.

[Gregs71]

Alright guys, I got the dist. cap off today. When I tried to move the shaft I could feel it moving ever so slightly. But it didn't fee like the upper part of the shaft was moving. It seemed to feel like it moved at the bottom of the dist. As if the top of the dist shaft was a pivot point, and it was the bottom that was moving.

Should there be any movement whats so ever in that shaft?? I am going to remove the module and have it tested in the mean time.

Here is a pic of the dist cap and rotor? I think it needs replacement. It seemed like it wore out a wide area on the rotor.
http://s28.photobucket.com/albums/c2...lverado015.jpg
http://s28.photobucket.com/albums/c2...lverado017.jpg
Can you tell anything else from the pics?
Let me know what you guys think?

Greg

ps- James, ChevyTech-- I never changed the o2 or egr. You guys seemed confident it was coming from the dist, so I am checking that first.

[Boog]

I'll just throw this out there. A mid 80's Caprice came in with the same symptoms. That one turned out to be 2 bad spark plugs. The porcelain insulator was cracked around the center electrode. They were AC plugs but I've never seen it before or since.

[rfmaster]

A lean condition can be caused by a low fuel pressure. TBI system requires 11 to 14 PSI to operate properly. Checking dizzy shaft for worn bushings is equally important.

//RF

[ChevyTech]

Being this problem only happens after the truck warms up, and all the popping in the intake and exhaust, leads me to believe it is an ignition problem.

I would think if the problem was fuel pressure, it would happen all the time, not only warm. All the popping does not sound like a fuel problem to me.

I the problem was a mechanical or valve problem, it should have at least some symptoms when cold.

You could try looking at the timing with a timing light cold and warm and see if you find anything odd.

Not hearing the truck run myself makes it hard to guess what is wrong, but if it is popping badly, I would try a distributor module even if the old one tested to be ok.

[Gregs71]

ChevyTech,

I think you were right on!!

I just got back from autozone where they tested the old module (in the dist) and it tested bad. I don't know what it means but it failed in the sat. / field portion of the test. Not sure what that means exactly, but thats what failed.

I will put the new one on and let you guys know if it solves the problem.

Greg

[Gregs71]

Well, went for a test drive. It is better than it was before!!! It backfired once or twice, but nowhere near the force it did before. It feels like it needs a tune-up.

I think it needs plugs/wires, cap/rotor to completely fix the issue. I will replace that stuff this week and let you guys know how its going.

one more question. Anybody ever here water sloshing a round from inside the cab. I can't believe I'd hear the sloshing of the gas tank. It does not sound like its coming from the doors. Can anybody think of any breather holes I could check for water, maybe in the cowl area??????

Greg

[ChevyTech]

After you do this work if there is ANY backfiring I suggest doing more testing begining with testing the engine compression to rule out a mechanical cause.

If no problem is found but some backfiring continues I would test the fuel pressure with the engine running. These TBI trucks have no fuel pressure test port.

[Gregs71]

Well this am, I went out to do a compression check on the truck. When I removed the air cleaner (to get to the dist. to disconnect it) I saw a vacuum line that was laying there on the intake. It was the vacuum line on the right (front dr) side of the carb and goes to the charcoal canister which is behind the dr side headlights.

Well I hooked it up and decided to test drive the truck. After warming up, the truck seemed to drive just fine.

Somehow, it must have come disconnected. I guess this is what contributed to the backfiring after I fixed the distributor module???

Does this make sense?

Greg

[ChevyTech]

When an engine is running lean it is more prone to backfiring in the intake.

A vacuum leak can make a TBI system lean in “open Loop”, which is when it is cold before the system uses to o2 sensor to monitor the combustion. When the engine is warm and in “closed loop” the system will add more fuel when it detects the engine is running lean. There is a short term and a long term fuel trim that takes place. The long term mixture change is very slow, but the short term happens much more quickly. The system can only compensate so far, so if there is a large leak it will help but may not be able to get the mixture perfect.

A large vacuum leak will make a TBI engine idle to fast. There is a limit to how far the IAC can compensate.

Being the engine was back firing, it could have blown off the vacuum line you found disconnected.

Sounds like you made good progress being is seemed to drive just fine.

Drive the truck a bit so the ECM can relearn the fuel trim tables and the IAC position and you may find it runs even better.

[Gregs71]

Well, I thought I had solved my problem, but driving a round town there was a small backfire coming from what I would say is at the intake. If I drive it easy, there is no problems. If I try to accelerate, even at half throttle it backfires.

I have dual tanks and drove on both. It did it with both tanks, so I am thinking that it is not the fuel pump (what are the odds of both going out).

Could it be the o2 sensor, EGR, etc.

The backfiring is no where near as bad as before I replaced the dist. module.
I guess I will do a compression check to make sure its ok.

Help me out guys.

Greg