2.8 V6 idle surge and hard starting

PEAR69

MECHANIC
1986 CADILLAC CIMARRON
55,000 MILES

Whether the engine is cold or warmed up, the engine is hard to start. I have to press the gas pedal to get it to start (like as if it had a carb.). It has MPI fuel injection. 95% of the time it will stall if I don't rev it up a little.
When it warms up a little, the idle surges for a few seconds and then returns to normal. It does this all the time - after engine is warm - every minute or so.
I smell excess fuel in the exhaust after it starts and when it is surging. The exhaust has NO restrictions and Cat is not blocked (don't ask, just trust me there, lol).
Fuel pressure holds steady at 40 psi - does not fluctuate with idle surge.
I have tested the TPS .48v closed 4.7v wide open operates smooth
Replaced distributor with new one with all new parts. Timing is perfect. Plugs + wires are new. Coil is good.
Replaced IAC valve - PCV valve - O2 sensor. No vacuum leaks.
Disconnected the IAC valve, MAF sensor, IAT sensor, TPS sensor, MAP sensor, TCC module - separately - one at a time, during idle surging and the surging did not change. Disconnected battery for 20 minutes and no change in problems.
Something seems to be causing the engine to become too rich with fuel. Any ideas?

5 Replies

HMAC300

MECHANIC
48,601 POSTS

Fuel pressure should 41-47 psi. Clean throttle plate with choke cleaner on both sides and check map sensor to see if there is a vacuum leak around wher eit plugs into the intake. The gasket may haveeaten away over time. Also your cold start injector may not be working. Have your pressure regulator checked as well it may be screwed upor the vacuum line leading to it is soft and sucking together. Scan for codes to see if there are any.

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Tuesday, April 10th, 2012 AT 1:28 PM

PEAR69

MECHANIC
1,482 POSTS

Thanks for the reply. I think I found some of the problems, but I just want to confirm. I went to clean the TB, and when I disconnected the vacuum line that goes to the fuel pressure regulator, fuel started pouring out of the vacuum line. Is that an indication of a failed FPR?
Also, I went through the test procedure as per the GM Service Manual for the cold start valve/and injector. The "valve" (as they call it - I call it another coolant sensor) failed the ohms test. When the engine is cold the 852 terminal has 150 ohms to ground -- the spec says less than 20 ohms. That "valve" is around $100.00, so I am wondering if you know if I can get away with disconnecting it for a little while?
I am going to donate as soon as I press send. Thanks for your time I really appreciate it!

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Wednesday, April 11th, 2012 AT 11:03 PM

HMAC300

MECHANIC
48,601 POSTS

Well those issues may clear up when your replacethe fpr. It may be that is'just loaded up and giving high resistance. It also depends on what temperature it was when you tested it if the engine was warm it will be higher. Yuo sholdn't have any fuel in the vacuum line so i'd say the fpr was bad.

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Thursday, April 12th, 2012 AT 1:01 AM

PEAR69

MECHANIC
1,482 POSTS

Changed the FPR and the problem is solved! Upon inspection of the old FPR, the seal failed causing fuel to be 'vacuumed' up into the throttle body, thus causing the surge when the vacuum was applied to the FPR. When engine was off, the residual fuel pressure forced fuel into the intake, through the vacuum line, thus causing the hard starting problem.
The cold start injector 'coolant sensor' (or 'valve' as GM describes it) still fails the ohm test, but I'm not changing it until it gets cold outside again. This 'valve' controls the cold start injector by sensing the coolant temperature. It never sees or touches fuel, so I'm just leaving it disconnected because the engine starts and runs perfect now. Thanks hmac300, how do I leave feedback?

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Friday, April 13th, 2012 AT 3:56 AM

HMAC300